refactor: update AppState references to use state module

- Change all instances of AppState to reference the new state module across multiple files for consistency.
- Clean up imports in auth, books, index_jobs, libraries, pages, search, settings, thumbnails, and tokens modules.
- Simplify main.rs by removing unused code and organizing middleware and route handlers under the new handlers module.

apps/api/src/state.rs

@@ -0,0 +1,61 @@
+use std::sync::{
+    atomic::AtomicU64,
+    Arc,
+};
+use std::time::Instant;
+
+use lru::LruCache;
+use sqlx::{Pool, Postgres, Row};
+use tokio::sync::{Mutex, Semaphore};
+
+#[derive(Clone)]
+pub struct AppState {
+    pub pool: sqlx::PgPool,
+    pub bootstrap_token: Arc<str>,
+    pub meili_url: Arc<str>,
+    pub meili_master_key: Arc<str>,
+    pub page_cache: Arc<Mutex<LruCache<String, Arc<Vec<u8>>>>>,
+    pub page_render_limit: Arc<Semaphore>,
+    pub metrics: Arc<Metrics>,
+    pub read_rate_limit: Arc<Mutex<ReadRateLimit>>,
+}
+
+pub struct Metrics {
+    pub requests_total: AtomicU64,
+    pub page_cache_hits: AtomicU64,
+    pub page_cache_misses: AtomicU64,
+}
+
+pub struct ReadRateLimit {
+    pub window_started_at: Instant,
+    pub requests_in_window: u32,
+}
+
+impl Metrics {
+    pub fn new() -> Self {
+        Self {
+            requests_total: AtomicU64::new(0),
+            page_cache_hits: AtomicU64::new(0),
+            page_cache_misses: AtomicU64::new(0),
+        }
+    }
+}
+
+pub async fn load_concurrent_renders(pool: &Pool<Postgres>) -> usize {
+    let default_concurrency = 8;
+    let row = sqlx::query(r#"SELECT value FROM app_settings WHERE key = 'limits'"#)
+        .fetch_optional(pool)
+        .await;
+
+    match row {
+        Ok(Some(row)) => {
+            let value: serde_json::Value = row.get("value");
+            value
+                .get("concurrent_renders")
+                .and_then(|v: &serde_json::Value| v.as_u64())
+                .map(|v| v as usize)
+                .unwrap_or(default_concurrency)
+        }
+        _ => default_concurrency,
+    }
+}